Someone who takes advantage of situations and people to get what they want. They put material and superficial things above people, and their wants over the needs of others. In a way, many politicians are also opportunists because they take advantage of the socially downtrodden to shove socialism and more government down our throats.

On an average day to day basis, an opportunist is a person who takes immediate advantage of an opportunity to achieve a goal and doesn't even think about the morals, ethics, principles or consequences of that action. They tend to put expediency over morality.

Usually describes desperate people who have never gotten any and dumb criminals.

That girl had a one night stand with some guy she met off the street and got knocked up. She is such an opportunist.

Not to mention a slut.
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What was that dude thinking when he tried to steal that laptop? There were security camera's everywhere! What a dumb opportunist.